1. Field of the Invention
The present invention relates to a deposition-film-forming apparatus and a gas feeding device. In particular, the present invention relates to a gas-feeding device which is suitable for feeding an organometallic compound-containing starting gas or the like in a thin-film deposition apparatus by the chemical vapor deposition (CVD) method.
2. Related Background Art
A chemical vapor deposition method using an organometallic compound is employed widely for the deposition of a metal thin-film or a semiconductor thin-film of III-V groups. The widely used organometallic starting materials, trimethylgallium (TMG) and trimethylaluminum (TMA), are liquid at room temperature. By the flow of a carrier gas like argon, which is introduced through a tube inserted into the liquid organometallic compound, the liquid organometallic compound is transported in a vapor state to a reaction vessel constituting a space for forming the deposited film.
FIG. 7 illustrates a conventional gas feeding device for transporting a gas (a starting gas) containing an organometallic compound. An organometallic compound 2 is stored in a liquid state in a metallic container 1. A carrier gas 6, such as argon, is blown as bubbles into the liquid through a metal pipe 41 which is inserted into the organometallic compound, 2. The organometallic compound brought into the bubbles in a saturated vapor state, is transported as the starting gas 7 to a reaction vessel (not shown) through a pipe 42 which is not inserted into the liquid. For example, with an inserted pipe 41, having a diameter of 1/4 inch and having a carrier gas flow rate of from 1 to 100 sccm, an organometallic compound such as TMG and TMA can be transported in an amount corresponding to the saturated vapor pressure in the carrier gas that is outgoing from the outlet pipe 42.
However, for improving productivity, namely for constituting a deposition-film-forming apparatus which is capable of depositing a thin film on a plurality of substrates at the same time, a large amount of starting gas is correspondingly required. If a large flow rate of the carrier gas, as much as from 1 to 10 l/min, is introduced through one metal pipe 41, as shown in FIG. 7, the organometallic compound will not be saturated sufficiently in the bubbles, so that the quantity of the transported organometallic compound will not increase even if the flow rate of the carrier gas is increased. In particular, for a viscous organometallic compound, the bubbles formed by the metal pipe 41 becomes large in size, causing a pulsation of a discharged gas flow, or in an extreme case, the bubbles may come to join together to form a tubular gas flow path which reaches to the gas layer above the liquid surface. For example, with a metal pipe 41 having 1/4 inch diameter, the carrier gas flow path may change at a flow rate of around 100 cc/min from a bubble form to a tubular form, resulting in a disproportionality between the of the amount of the transported organometallic compound and the flow rate of the carrier gas.
Accordingly, for transportation in a high flow rate, use of a plurality of containers for containing the organometallic compound may be considered. For example, for a required flow rate of the carrier gas of from 1 to 10 l/min, 10 to 100 containers are required on the assumption that 100 cc/min of the carrier gas can be introduced per container. In such a case, problems may be involved because of a size increase in the apparatus, a rise in the cost, and complications in maintenance.
On the other hand, Japanese Patent Laid-open Application No. sho-62-33769 discloses the perforation of a number of holes at the tip of the metal tube. This may be effective for an organometallic compound having a low viscosity such as TMG, but cannot always be effective for an organometallic compound having a high viscosity.
Japanese Patent Laid-open Application No. sho-62-207870 discloses the installation of an ultrasonic wave generator having a magnetostrictive oscillator coupled onto an organic metallic compound containing container. In an example thereof, a metal pipe for introducing a carrier gas is inserted into the compound in a gas phase. However, the generation of mist by the ultrasonic effect cannot be expected because the container generally contains little of the gas which causes cavitation. Although a description is found in the patent publication that the metal pipe for the carrier . introduction may be inserted into the organometallic compound, a sufficient cavitation effect cannot be expected in viscous organometallic compounds because bubbles are not readily generated when the carrier gas has a high flow rate, as mentioned above.
Further, for the case of organometallic compounds having a low viscosity like TMG, the above-cited Japanese Patent Laid-open Application No. sho-62-207870 shows a method for transporting TMG effectively at a high flow Pate. This method, however, is not so effective for organometallic compounds like DMAH (dimethyl aluminum hydride) having a high viscosity. This is because no measure is taken during the introduction of a carrier gas for the formation of minute bubbles in the organometallic compound.
Japanese Patent Laid-open Application No. sho-60-131973 discloses a method for vaporizing a liquid organometallic compound by bubbling the gas in such a manner that the gas is ejected through a bubble-forming device having a plurality of gas-ejecting holes at the tip of the gas ejection portion. Even with this method, however, there may be some instability during the feeding of the gaseous organometallic compound for the vaporization of a large amount of the organometallic compound for rapid formation of a deposition film because bubbles which are caused by the introduction of a large amount of gas, combine mutually to form even larger bubbles that splash out of the liquid organometallic compound the splashing being caused by the bursting of the larger bubbles, and the resulting accumulation of the liquid in a feed pipe.
As discussed above, there has been no satisfactory gas feeding device for feeding a large amount of a starting gas employing an organometallic compound having a high viscosity.